The Cleaning and Lacquering of Museum Silver

by Glenn Wharton

Editor's Note: I asked Glenn to prepare a pared down version of
the lecture on silver he presented at the WAAC Conference in
Yosemite.

The following is a brief summary of the procedures that are used
to clean and lacquer silver. Please remember that each object has
its own story to tell, and this summary does not attempt to cover
the wide variety of problems one faces in conserving silver
artifacts.

Tarnish Removal:

In general, cleaning with acidified thiourea is preferred over
abrasive cleaning. Thiourea chemically dissolves silver sulfide
(tarnish). Tarnish on gilt surfaces should always be removed
chemically, since abrasives will wear away the thin layer of gold
over time. The intensive washing which follows the thiourea
treatment may prevent its use on silver with associated organic
materials such as ivory or wood.

After careful inspection to insure that no lacquer is present on
the object, the surface should be mechanically cleaned of surface
grime and old polish residue. This can be achieved with wood
skewers, stencil brushes or ultrasonics. Next, the tarnish can be
removed with acidified thiourea, using either cotton swabs or
stencil brushes. The solution takes approximately one minute to
dissolve the tarnish. After several minutes of light rubbing or
brushing, rinse the silver to avoid redeposition of tarnish.
Redeposited tarnish can be very difficult to remove. Swabs must be
tossed and brushes must be rinsed before cleaning the next area to
prevent contaminating the thiourea solution.

After tarnish removal, the object must be intensively washed.
Following a thorough sudsing with deionized water and neutral
detergent (e.g. Triton X-100), rinse the object for one hour in a
tub with slowly running deionized water. After a final sudsing and
rinsing, the object should be dried with filtered compressed air,
clean diapers, and reagent grade ethanol. The surface of the silver
will have a characteristic micro-roughness after thiourea cleaning,
which can be removed by light buffing with a clean diaper.

Thiourea is a suspected carcinogen, and should be handled with
extreme care. Protective gloves and toxic vapor masks must be worn
at all times. Conservators should consult the manufacturers and
distributors of thiourea for proper safety precautions. [Editor's
note: Thiourea is on the Governor's list of Chemicals known to the
State of California to Cause Cancer and is therefore covered by the
Safe Drinking Water and Toxic Enforcement Act of 1986 (proposition
65). Warning signs are required for thiourea as of January 1989 and
the disposal restrictions will go into effect in August 1990, see WAAC
Newsletter, Vol. 10, no. 2 (May 1988), for further
information.]

Because of the health hazards associated with thiourea, abrasive
cleaning is sometimes preferred. This can be achieved with
precipitated calcium carbonate in a slurry of deionized water to
which several drops of Triton X-100 have been added. Cotton diapers,
cotton swabs, and stencil brushes are used to rub the abrasive
slurry on the silver. If precipitated calcium carbonate fails to
remove the tarnish then gamma alumina (0.05 microns) may be tried.
For stubborn tarnish, alpha alumina (0.3 microns) may be used. The
aluminas are more abrasive however, and will remove more silver
during the cleaning process. They also impart a higher reflectance
to the surface, which may be inappropriate for historical
silver.

Lacquering:

After complete drying, the object should be thoroughly degreased
with reagent grade ethanol, acetone or methyl ethyl ketone. Gloves
should be worn at all times when handling silver. Grease from
handling will prevent lacquer from adhering to the surface. There is
a current debate among conservators whether cellulose nitrate or
acrylic lacquers are preferable. Among the products commonly used
are Agateen No. 27 (cellulose nitrate) and Acryloid B-72. The
lacquer may be applied either by brushing or spraying. The
application should always consist of two layers, to insure complete
coating.